The present invention relates to a lock-up clutch utilized in a torque converter providing a direct drive between the impeller and turbine of the torque converter to improve the fuel efficiency of the transmission. The lock-up clutch assembly includes a piston or clutch plate located in the torque converter between the forward wall of the housing and the turbine. The piston plate is operatively connected to the transmission input shaft through the turbine hub, and a vibration damper may be inserted between the piston plate and the turbine hub to dampen any torsional vibrations occurring from the vehicle engine or other driving means.
The piston plate and associated structure is designed to axially reciprocate relative to the turbine hub and transmission input shaft so that an annular friction facing on the piston plate engages a mating friction surface in the torque converter housing. Actuation of the piston plate is caused by a pressure differential in the chambers formed on the opposite sides of the piston plate due to the pressure increase resulting from operation of the torque converter. To disengage the lock-up clutch, the pressure differential must be equalized or reversed to allow the piston plate to retract, which may be accomplished by providing a source of pressure acting on the opposite side of the pressure plate.
If the vehicle operator lifts his foot from the accelerator pedal, thus decreasing the engine revolutions, a quick and positive disengagement of the clutch piston plate is desirable; especially for a vehicle powered by a diesel engine. The use of a pressure source to retract the plate and/or a one-way clutch in the operative connection between the piston plate and turbine hub has not satisfactorily solved the problem of disengagement of the clutch, however, the pressure relief valve of the present invention appears to obviate the problem.